Differentiation of Bacillus subtilis to form endospores was investigated by biochemical and cytological techniques. The following major results were obtained: 1. Manganese is required for sporulation only because it is needed as cofactor for phosphoglycerate mutase. 2. Sporulation can be induced by decoyinine, an inhibitor of GMP-synthetase, even in certain developmental (is equal to cacoenic) mutants normally unable to sporulate. 3. Following the complete enclosure of a forespore by a double membrane the mother cell loses the ability to multiply; apparently, forespores secrete a compound that causes the deterioration of the mother cell.